The present invention relates generally to methods and apparatus for removing adherent layers from substrates, and more particularly to methods and apparatus for removing residues from semiconductor wafers, including post-etch, post-ash, and other post-process residues.
The fabrication of integrated circuits and other devices on semiconductor wafers depends on the photolithographic patterning of successive layers of materials applied on and into the wafer. In each photolithographic step, a layer of photoresist is applied to the wafer, soft baked, and patterned by exposure to radiation through a precisely aligned mask. Such exposure alters the solubility of the resist material in a particular solvent, thus allowing selective removal of the resist in accordance with the pattern defined by the mask. After the subsequent fabrication steps are completed, it becomes necessary to remove the remaining insoluble photoresist to permit further fabrication. In some cases, the photoresist may be rendered insoluble to common solvents by the fabrication step which has been performed. For example, ion implantation, radiation accompanying plasma etching, and any other process raising the wafer temperature above 150.degree. C. to 200.degree. C. for a significant period of time, will frequently cause the photoresist material to become heavily cross-linked, making it particularly difficult to remove.
During photolithography, a photoresist mask is formed on a semiconductor substrate. A hard, cross-linked polymeric crust is typically formed at the top of the photoresist and residues are formed along the walls of any trenches or vias during subsequent processing. For instance, impurities, such as P, B, and As, may be bonded chemically with the photoresist polymer during ion implantation, and the surface layer of the photoresist mask will change in quality and become a very hard layer known as a carbonized layer. The photoresist polymer inside the photoresist mask beyond the reach of impurities will remain as an unchanged layer.
In addition to cross-linking, contamination of a photoresist layer during a wafer fabrication step can also reduce its solubility. For example, when photoresist is used for patterning a silicon dioxide or aluminum layer during plasma etching, the organic photoresist material may become contaminated by silicon, aluminum, or other inorganic materials. Such contaminated photoresists are frequently refractory to normal solvent removal.
Some techniques of removing refractory photoresists involve thermal and photochemical oxidation of the photoresists. Such oxidation typically requires elevated temperatures, which can cause undesired diffusion within the wafer, and can cause sputtering of the metallic components made of, e.g., aluminum and titanium nitride. Oxidation of such metallic components will form metal oxides such as aluminum or titanium oxides, which are ceramic residues that are extremely difficult to remove. Others employ low temperature "ashing" in an oxygen plasma, which has the disadvantage that the plasma discharge required can result in damage to the wafer substrate. Wet oxidative stripping of insoluble photoresists has also been used. Such wet stripping techniques, however, often require temperatures above 150.degree. C. to be effective.
In U.S. Pat. No. 5,201,960 to Starov, which is incorporated herein by reference in its entirety, a densified fluid cleaning method for removing adherent matrices involves exposing the matrix to a vapor phase solvent to allow the solvent to penetrate the matrix and then condensing the vapor to physically disrupt the adherent matrix to promote fragmentation of the matrix and facilitate removal. The solvent typically used is ammonia. While this dry cleaning method has the advantages of lower cost, improved safety, and reduced environmental impact, it is sometimes not effective in completely removing post-etch or other post-process residues. For instance, the interaction between ammonia and the photoresist can be quite slow, making the removal of thick photoresist layers difficult. In cases where there is a stubborn polymeric crust on top of the photoresist, the stresses produced during interaction with ammonia may cause curling or rolling of the polymeric crust and the underlying photoresist, resulting in macroscopic flakes over the substrate surface. Thus, there is a need for more effective methods and apparatus for removing adherent matrix layers such as post-etch or other post-process residues.